1, Field of the Invention
The present invention is directed to a process for the production of cold setting, flexible polyurethane molded foams using increased amounts of water at an isocyante index of less than 70.
2. Description of the Prior Art
Cold setting, flexible, polyurethane molded foams are widely used inter alia, in the motor vehicle industry and the furniture industry. The quality of the flexible, polyurethane molded foams conventionally used for upholstering seats and arm rests depends for the most part on their gross density. Regardless of the nature of the basic raw materials (polyols/isocyanates) used for the preparation of the cold setting molded foams, it has been found advisable and practical for certain uses and the attendant requirements, especially with regard to the long term durability in use, to ensure that the foams reach a certain level of gross density.
The gross density obtained is mainly determined by the water content in the combination of raw materials. Typical water contents are up to 5 parts by weight (generally 2.5 to 3.5 parts by weight) per 100 parts of polyol, depending on the particular level of gross density required.
It has been shown that the lower gross densities frequently required for the purpose of reducing weight and cost (although this depends on the particular purpose for which the foam is to be used) can only in exceptional cases be achieved by increasing the water content above the aforesaid quantities. The reason for this lies in the fact that as the proportion of water increases (above 3.5 parts by weight per 100 parts by weight of polyol), the catalytic adjustment of the chemical reaction (polymer formation/gas formation) which is essential for the foaming process becomes more difficult and the latitude allowable for the process is greatly restricted. Apart from problems relating to the nature of the skin (mold temperature/mold release agent) and the stability of the foam, the range of isocyanate indexes allowable is also restricted. For these reasons, any desired or necessary lowering of the gross density is preferably brought about by the addition of physical blowing agents (e.g. halogenated hydrocarbons). This method is state of the art and is in principle applicable to all cold setting molded foams.
When pure MDI is used and especially also when MDI prepolymers are used, the quantity of CO.sub.2 evolved (per unit quantity of water) must be capable of foaming up a larger quantity of raw material than in the case of TDI (NCO=48.3%) owing to the comparatively low NCO content of MDI (about 32% or about 25% or less). Therefore, it has frequently been necessary to add alternative blowing agents in order to obtain at least a reasonably satisfactory level of gross density for the given use or purpose. This procedure, however, cannot be recommended in view of the environmental pollution and the high cost.
In order that the conventional ranges of gross density of PUR foams may be further reduced when pure TDI, TDI mixtures or corresponding prepolymers are used, it has previously also been necessary to use additional, alternative blowing agents as in the case of MDI. This method is also inadvisable for reasons of environmental protection.
It has now surprisingly been found that atypically high quantities of water of more than 5.0 parts by weight per 100 parts by weight of "basic polyol" can easily be worked up and consequently gross densities as low as 15 kg/m.sup.3 may be obtained in cold setting, flexible, polyurethane molded foams if instead of employing an isocyanate index within the conventional range (80 to 120), a prohibitively low isocyanate index of less than 70 is used for foaming.
It is known from GB-PS 892,776 that rigid or semi-rigid polymer foams may be prepared by the reaction of 100 parts by weight of a monomeric organic polyisocyanate with about 1 to 30 parts by weight of water in the presence of a surface active agent and a catalyst, preferably in the presence of 5 to 30 parts by weight of a polyfunctional compound containing two or more than two isocyanate reactive groups. However, it is clear from the claims and from the examples of the British patent that the "catalysts" used therein are inorganic catalysts such as potassium acetate or sodium hydroxide so that the foaming process is dominated by trimerization reactions such that the low isocyanate indexes according to the present invention are not used.
The PUR flexible molded foams to be produced according to the present invention also differ from the rigid and semi-rigid foams obtained according to GB-PS 892,776 in that they have different molding characteristics. It is apparent, for example, from the measurements of rigidity obtainable from the deformation diagrams that the PUR flexible, molded foams produced according to the present invention have a higher elasticity (= capacity for more rapid restoration after subjection to a load) and a lower level of rigidity. Both of these features are advantageous for the particular requirements of these products in practical use. A characteristic magnitude expressing this feature is the relative energy absorption after 70% compression = H 70 (see tables on pages 17 and 18).
It is further known from GB-PS 1,133,691 that flexible polyurethane foams can be obtained by the reaction of polyisocyanates such as diphenylmethane diisocyanate with higher molecular weight polyols in the presence of 1.5 to 5.0 parts by weight of water based on 100 parts by weight of the high molecular weight polyol. The preparation of these foams is carried out in open molds. However, it was found that the application of such higher amounts of water as blowing agent in the preparation of these foams in open molds often led to a self-ignition of the foam during storage. Also, these foams were often scorched. It was therefore unexpected that these disadvantages could be avoided in the preparation of the foam in closed molds if even higher amounts of water were applied as blowing agent.